Precision Small Scattering Angle Measurements of Elastic Proton-Proton Single and Double Spin Analyzing Powers at the RHIC Hydrogen Jet Polarimeter

The Polarized Atomic Hydrogen Gas Jet Target polarimeter is employed by the Relativistic Heavy Ion Collider (RHIC) to measure the absolute polarization of each colliding proton beam. Polarimeter detectors and data acquisition were upgraded in 2015 to increase solid angle, energy range, and energy resolution. These upgrades and advanced systematic error analysis along with improved beam intensity and polarization in RHIC runs 2015 ($E_{\mathrm{beam}}=100\mathrm{GeV}$) and 2017 (255 GeV) allowed us to greatly reduce the statistical and systematic uncertainties for elastic spin asymmetries, $A_{\mathrm{N}}(t)$ and $A_{\mathrm{NN}}(t)$, in the Coulomb-nuclear interference momentum transfer range $0.0013<-t<0.018{\mathrm{GeV}}^2$. For the first time hadronic single spin-flip $r_5$ and double spin-flip $r_2$ amplitude parameters were reliably isolated at these energies and momentum transfers. Measurements at two beam energies enable a separation of Pomeron and Regge pole contributions to $r_5(s)$ and $r_2(s)$, indicating that the spin component may persist at high energies.

Figure 1

A schematic view of the HJET recoil spectrometer consisting of eight silicon detectors with 12 vertically oriented strips (readout channels) each. The distance between beams is $\sim 2\mathrm{mm}$.

Figure 2

Measured normalized asymmetries in RHIC $\mathrm{Run}15$ (100 GeV) and $\mathrm{Run}17$ (255 GeV). The fit energy range is $1.9<T_R<9.9\mathrm{MeV}$ for the 255 GeV $a_n^{\text{jet}}(T_R)$ and $0.7<T_R<9.9\mathrm{MeV}$ for the other graphs. The fit parameter ${\tilde{\alpha }}_5$ is defined as ${\tilde{\alpha }}_5=⟨P⟩{\alpha }_5$.

Figure 3

Double spin asymmetry $a_{\mathrm{NN}}$ measured at HJET. The fit used values of $P_{\mathrm{j}}$ and $P_{\mathrm{b}}$ from the single spin analysis.

Figure 4

The Reggeon contributions $\mathcal{R}(s)=R_{\mathcal{R}}(s)+i{I}_{\mathcal{R}}(s)$ to Eq. (22) defined by the AU-$\mathrm{L}\gamma =2(\mathrm{T})$ fit of Ref. [29].

Figure 5

$\mathrm{\Delta }{\chi }^2=1$ correlation ($\text{stat}+\text{syst}$) contours for $r_5$ and $r_2$. Filled ellipses mean statistical error only. The HJET extrapolations to 200 GeV are labeled 3 and 4. The STAR Collaboration result [11] for $r_5$ was changed by us using Eqs. (5)–(8).

Figure 6

Elastic $pp$ analyzing powers $A_{\mathrm{N}}(t)$ and $A_{\mathrm{NN}}(t)$ measured in this work. The filled areas correspond to $\pm {\sigma }_{\mathrm{stat}+\mathrm{syst}}$. For $A_{\mathrm{N}}(t)$, the dashed lines refer to the expected analyzing powers if $r_5=0$.